A SP game can be started and stopped when you want.
A SP game let you play at your pace, without being worried about taking too long to play.
A SP game let you play calmly, without the stress of playing against someone.
In MP game, I've componction about being mean to a player-controlled civ. Anyway, I prefer to be allied with the human than enemy.
In MP, there is less "roleplaying", as you're playing with/against a person who knows the rules of the game, and have more tendencies to use all the trick rather than to act like a faction leader.
For all these reasons, I'll play MUCH, MUCH more SP than MP.